Responses in muscle afferent fibres of slow conduction velocity to contractions and ischaemia in the cat.

Abstract

This study examined to what extent muscle receptors with slowly conducting afferent fibers (group III and IV) are activated by muscular contractions of moderate force, and what kind of muscle afferents could mediate the pain of ischemic exercise. In chloralose-anesthetized cats, the impulse activity of single afferent units from the triceps surae muscle was recorded from dorsal root filaments during muscular contractions with intact blood supply and after occlusion of the muscle artery. Two types of responses were observed to contractions without muscular ischemia. One was characterized by sudden onset and a graded response amplitude to contractions of increasing force. In most cases stretching the muscle was also an effective stimulus. Units showing this response behavior were labeled c.s.m (contraction-sensitive with mechanical mechanism of activation). The other response type had a more delayed onset and often outlasted the exercise period [because of the unknown mechanism of activation, units of this kind were labeled c.s.x]. The proportion of c.s.m receptors was significantly higher among group III than among group IV units. During ischemic contractions of comparable force the c.s.m and c.s.x receptors exhibited an unchanged or a decreased response amplitude. Under these conditions another receptor type (N, for nociceptive) was activated which did not respond to contractions with intact blood supply. Vigorous activations during ischemic work were only observed in group IV receptors. The majority of the 131 group III and IV units tested did not respond to contractions at all. These contraction-insensitive (c.i.) endings probably comprised different receptor populations (nociceptors, thermoreceptors, low-threshold mechanoreceptors). The various central nervous effects of muscular exercise without ischemia which are known to be due to raised activity in thin muscle afferents (e.g. cardiopulmonary adjustments, spinal locomotor reflexes) are probably produced by the c.s.m and c.s.x types. The pain of ischemic contractions is most likely mediated by the N receptors most of which possess non-myelinated afferent fibers.